Electrically conductive ceramics and their use in fiber charging apparatus

ABSTRACT

A conductive ceramic composition is disclosed which consists essentially of alumina, chromia, and magnesia, and is suitable for use as target electrodes in electrostatic fiber charging applications. Ceramics are disclosed which have exhibit volume resistivities of 10 12  ohm-cm or less at 20° C., and have excellent electrical stability and superior mechanical properties; and an electrostatic charging apparatus is disclosed which employs a ceramic electronic conductor having a volume resistivity of from about 1×10 7  to 5×10 10  ohm-cm and a hardness of at least 5 GPa.

This is a division of application Ser. No. 08/282,939, filed Jul. 29,1994.

FIELD OF THE INVENTION

This invention relates to electrically conductive compositions and theiruse, and more particularly to electrically conductive ceramiccompositions and their use.

BACKGROUND

Compositions having selective electrical conductivity have beendeveloped for various purposes. For example, U.S. Pat. No. 3,578,739discloses that a composition having a resistance measured at 65° C. atbetween 1×10⁶ and 1×10¹⁰ ohms is useful for a covered surface of atarget electrode employed in an apparatus for electrostatically charginga continuous fibrous material (e.g., polyethylene) being forwarded in alinear path. Certain carbon-filled polymeric covering compositions for ametal electrode base are disclosed as suitable.

The electrostatic charging of flash-spun polyethylene may beaccomplished by corona charging. Target electrodes used in theseprocesses should have the ability to withstand corona potentials used inthe charging process, should be suitably resistive to collect the coronacurrent while inhibiting back-corona, should be structurally sound, andshould be sufficiently hard to undergo mechanical cleaning. There isinterest in developing ceramic materials for this application.

Historically, the invention of the transistor initiated the developmentof high purity single crystal semiconducting materials such as germaniumand silicon. Today these materials are prepared with unparalleledperfection. In contrast, the development of electronic polycrystallineceramics has been much more placid. The majority of polycrystallineelectronic ceramics are produced using less defined synthesis andmanufacturing processes. The resulting devices are often multicomponentand multiphase and contain significant microscopic and macroscopicdefects. Undesirable impurity levels are orders of magnitude greaterthan in typical single crystalline semiconductors. In order to achievepolycrystalline perfection, processing and impurities levels must becontrolled to avoid unwanted second phases, mixed valence conductioneffects, and the segregation of impurities and second phase formationsat grain boundaries.

The electrical conductivity of ceramic materials can encompass a widerange of values ranging from insulators to semiconductors to those ofmetallic conductors. Electrical conductivity in metals occurs by themovement of free electrons and is the result of bonding in the bulkcrystal structure. In non-metallic materials such as ceramics,electrical conductivity can involve both electronic (migration of freeelectrons and holes) and ionic (migration of charged atoms) chargecarriers and is the result of atomic imperfections (point defects) inthe crystal structure or electronic imperfections (departure fromstoichiometry). Atomic point defects are variations from the perfectperiodicity of the crystal lattice. Four types of point defects areconsidered important in influencing conductivity: (1) vacancies (i.e.,atomic sites in the crystal lattice which are not occupied that in theideal crystal should be occupied); (2) interstitial atoms (i.e., atomswhich occupy sites that in the ideal crystal should not be occupied);(3) misplaced atoms (i.e., atoms which occupy sites that in the idealcrystal are assigned to atoms of a different type; for example, in an ABcrystal, a few A atoms may occupy B sites and vice versa); and (4)impurity atoms (i.e., impurity atoms which occupy normal host atom sitesor interstitial host sties). Other defects that commonly occur inmaterials are dislocations, grain boundaries, and surface defects thatinteract with point defects to affect electrical conductivity. There canbe considerable structural and compositional variations between the bulkcrystal and regions close to dislocations, grain boundaries, andsurfaces. These microscopic parameters often dominate electricalproperties.

In polycrystalline materials (which by definition are comprised ofindividual crystallites separated by grain boundaries) defects,impurities and second phases are known to segregate to grain boundariesand strongly influence electrical properties (see, e.g., W. D. Kingery,J. Am. Ceram. Soc., 57 (1974) 1-8). It has been clearly established thatgrain boundaries either act as paths of high mobility to enhanceelectrical conductivity or inhibit transport with respect to the bulkand reduce conductivity (see, W. D. Kingery, Advances in Ceramics,Volume 1, (L. M. Levinson, Ed.) American Ceramic Society, Columbus,Ohio, (1981) 1-22). Since the tendency is for solutes to segregate tograin boundary regions, the atomic feature that most often has thegreatest impact on electrical behavior in polycrystalline materials isthe grain boundary region.

It is generally accepted that to enhance conductivity in polycrystallinematerials, the level of impurity ions must be controlled and theformation of bulk and grain boundary second phases should be avoided.Silica is a particularly influential impurity since it is generallypresent as an impurity and has limited bulk solubility in ionic metaloxide ceramics. Silica segregates at grain boundaries forming amorphousfilms that often adversely affect electrical properties. Intrinsic grainboundary effects can, however, be used advantageously to createengineered electrical properties in polycrystalline systems that cannotbe obtained in single crystal systems. Varistors, thermistors, andbarrier layer capacitors are good examples of devices that exploit grainboundary effects to obtain their unique electrical properties.

Alumina has many desirable intrinsic physical properties includingmechanical strength, temperature resistance, chemical inertness, andelectrical resistance that are primarily determined by its crystalstructure. The electrical resistivity of alumina-based ceramics istypically greater than about 10¹⁵ Ωcm at room temperature, which makesit suitable for use as insulators and electronic substrates. Indeed,chromia-alumina polycrystalline samples without magnesium cationadditions are considered electrically insulating (i.e., their volumeresistivity, ρ, is generally greater than about 10¹⁵ Ωcm) andmagnesia-alumina polycrystalline samples without chromium cationadditions are also considered electrically insulating (i.e., their ρ isgenerally greater than about 10¹⁵ Ωcm). This is generally true attemperatures ranging from about 25° C. to 600° C.

Although alumina has desirable physical properties and is easilyfabricated into functional shapes by standard ceramic forming and firingmethods, its high electrical resistivity makes it unsuitable forapplications requiring electrical conduction. Numerous ceramics areelectrically conductive. However, most conductive ceramics haveundesirable physical characteristics such as poor mechanical or thermalproperties and are difficult to fabricate into useful shapes. A ceramiccomposition taking advantage of alumina's superior physical parameters,while being electrically conductive as well, would represent aparticularly desirable ceramic.

With regard to electronic conductivity in oxides, there are generallytwo ways to enhance conductivity in the bulk crystal: by the departurefrom stoichiometry, or by the introduction of impurity atoms into a hostlattice (controlled valence conduction). For alumina, conduction bydeparture from stoichiometry is unlikely because of alumina's highenthalpy of formation (1674.4 kJ/mol). Conduction from substitutionalimpurities is possible, however, though this has been shown to belimited at room temperature for alumina (see, F. A. Kroger, Advances inCeramics, Volume 10, (W. D. Kingery, Ed.) American Ceramic Society,Columbus, Ohio, (1984) 1-15). Thus, any room temperature enhancement ofelectronic conductivity in alumina-based ceramics must come from grainboundary effects.

The majority of commercially available aluminas are sintered via aliquid phase route through the use of additives such as oxides ofsilicon, calcium, sodium, and potassium, often added in the form ofminerals or clays. These additives enhance formation of siliceous liquidphases, and the presence of these viscous liquid phases during sinteringaid densification at relatively low firing temperatures. They also formglassy (siliceous) grain boundary films on cooling. Even whenliquid-forming additives are not used, sufficient impurities aregenerally present in the starting alumina powder to result in traceliquid formation upon sintering. Typical impurities include SiO₂, CaO,Fe₂ O₃, TiO₂, K₂ O, and MgO. Siliceous grain boundary films candeleteriously affect properties such as thermal and electricalconductivity by scattering conducting photons or electrons. Forelectronic conducting materials it is desirable to eliminate glassygrain boundary films to minimize scattering effects. Thus, of primaryimportance in developing an electrically conductive alumina is theelimination of any siliceous grain boundary films and unintended secondphases and unintended impurity ions that may segregate to grainboundaries.

SUMMARY OF INVENTION

This invention provides an apparatus for electrostatically charging acontinuous fibrous material being forwarded in a path from a source to acollecting means including a charged ion gun and an opposed groundedtarget electrode positioned on opposite sides of said path between saidsource and said collecting means, characterized by said target electrodehaving a ceramic surface facing said ion gun, said ceramic being astable electronic conductor having a volume resistivity of from about1×10⁷ to 5×10¹⁰ ohm-cm measured at the operating temperature, and aVickers hardness of at least about 5 GPa.

The present invention further provides a ceramic composition of matterconsisting essentially of oxides of Mg²⁺, Cr³⁺ and Al³⁺, wherein theatomic ratio of Cr³⁺ to Al³⁺ is within the range of from about 1000 toabout 100,000 ppm (i.e., there are from about 1000 to 100,000 Cr³⁺ atomsper million Al³⁺ atoms), and the atomic ratio of Mg²⁺ to Al³⁺ is withinthe range of from about 25 ppm to about 10,000 ppm (i.e., there are fromabout 25 to 10,000 Mg²⁺ atoms per million Al³⁺ atoms). Ceramic bodies(which may be formed by sintering such ceramic compositions wherein theMg²⁺ content is about 3 times the solubility limit of magnesia inalumina of the sintering temperature, or less at a sintering temperatureof about 1950° C., or less) are also provided. In accordance with thisinvention, the ceramic body may be formed by sintering in an oxidizingatmosphere to inhibit electrical resistivity. This invention provides asintered alumina-based ceramic composition of matter consistingessentially of oxides of Mg²⁺, Cr³⁺ and Al³⁺, wherein Cr³⁺ and Mg²⁺ arepresent in amounts sufficient to provide a resistivity of about 10¹²ohm-cm or less at 20° C. This invention further provides an electricallyconductive polycrystalline, solid solution alumina ceramic prepared by aprocess comprising the steps of preparing a liquid containing a mixtureof Mg²⁺ and Cr³⁺ cations by adding a Mg²⁺ -containing precursor and aCr³⁺ -containing precursor to a solvent; mixing with the liquid a highpurity alumina powder to obtain a well-dispersed slurry; evaporating theliquid from the slurry to form a dried powder containing the magnesiumand chromium cations and alumina; preparing a green powder compact fromthe dried powder; and heating the green powder compact in an oxidizingatmosphere to densify. In this ceramic preparation the Cr³⁺ is mixedwith said alumina in an amount sufficient to provide an atomic ratio ofCr³⁺ to Al³⁺ within the range of from about 1000 ppm to about 100,000ppm, and the Mg²⁺ is mixed with said alumina in an amount sufficient toprovide an atomic ratio of Mg²⁺ to Al³⁺ within the range of from about25 ppm to about 3 times the solubility limit of magnesia in alumina atthe sintering temperature.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 graphs resistivity versus chromium cation additions, at constantvoltage (10 kV), temperature (60° C.), magnesium cation content (500ppm) for a time of 60 minutes.

FIG. 2 graphs resistivity versus chromium cation additions at constantvoltage (10 kV), temperature (60° C.), magnesium cation content (700ppm) for a time of 60 minutes.

FIG. 3 graphs resistivity versus voltage for various chromium cationadditions at constant temperature (60° C.) and magnesium cation content(500 ppm).

FIG. 4 graphs resistivity versus magnesium cation additions at constantvoltage (10 kV), temperature (60° C.), chromium cation content (20000ppm) for a time of 60 minutes.

FIG. 5 graphs resistivity versus time for various chromium cationadditions at constant voltage (10 kV), temperature (60° C.), magnesiumcation content (700 ppm).

FIG. 6 graphs volume resistivity versus time for various magnesiumcation additions at constant voltage (10 kV), temperature (60° C.), andchromium cation content (20000 ppm).

FIG. 7 represents a complex impedance plot at 250° C. for a samplehaving 30000 ppm chromia, 500 ppm magnesia and no silica impurities.

FIG. 8 represents an Arrhenius plot for low and high frequencycomponents to grain boundary electrical conductivity for analumina-based sample having 30000 ppm chromia, 500 ppm magnesia, and nosilica.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention provides a ceramic composition of magnesia, chromia andalumina, that when sintered forms a polycrystalline, solid solutionalumina-based body that is electrically conductive at room temperature,has excellent electrical stability, and excellent mechanical properties.More specifically, by controlling the chromium, magnesium and impuritylevels and by avoiding undesirable segregation and second phaseformations at grain boundaries, superior room temperature semiconductingproperties are achieved in an alumina-based polycrystalline ceramic.

A feature of the electrically conductive compositions of this inventionis their excellent stability against voltage breakdown at high d.c.voltages, (e.g., 10 kV). Electrical instability generally isdemonstrated by an increase or drift in resistivity with time, whicheventually leads to voltage breakdown. Compositions may be prepared inaccordance with this invention with essentially no resistivity drift.Electrical stability is especially important in high voltage d.c.applications and is generally not obtainable in electronic ceramics tothe degree observed in the materials of this invention.

Ceramic bodies can be prepared in accordance with this invention withresistivities of about 10¹² Ω-cm or less at approximately 20° C. undera.c. conditions. Ceramic bodies can also be prepared in accordance withthis invention with resistivities of about 10¹² Ω-cm or less atapproximately 20° C. under d.c. conditions. By resistivity is meant theelectrical resistance per cm of an article of 1 cm² cross-sectionalarea, having units of ohm-cm. Resistivity is the inverse ofconductivity, and may also be referred to as bulk or volume resistivity.It can be determined, per Ohm's law, by the following equation:ρ=(V)(A)/(I)(t) where V is the voltage (in volts), A is the samplecross-sectional area (in cm²), I is the current (in amperes), and t isthe sample thickness (in cm).

By "essentially no resistivity drift" is meant that the resistivity of amaterial will not vary over time from an initial resistivity by morethan about 10%, under constant d.c. voltage of at least 10 kilovolts andconstant temperature up to at least 350° C.

In the compositions of this invention, magnesia (MgO) and chromia (Cr₂O₃) are added as substitutional additives in solid solution withalumina, avoiding the formation of silaceous grain boundary phases andsecond phase spinel (MgAl₂ O₄) to produce a ceramic body which iselectronically conductive. FIG. 3 shows that resistivity decreased asthe chromia content was increased. FIG. 4 shows that resistivitydecreased as the magnesia content was increased up to the solubilitylimit of magnesia in alumina; increasing the magnesia content beyond thesolubility limit did not appreciably affect resistivity. FIG. 6 showsthat concentrations near the solubility limit exhibit essentially noresistivity drift.

Chromia (Cr₂ O₃) is soluble with alumina across the chromia-aluminabinary mixture and can thus be added without the formation of additionalphases. Magnesia (MgO) has limited solubility in alumina and ispreferably added below its solubility limit to avoid the formation ofspinel and achieve a single-phase product. In addition to providingelectrical conductivity in the compositions of the invention, magnesiain alumina acts as a solid-solution sintering aid which promotesdensification, controls (retards) grain growth and has been suggested toredistribute silica impurities (see, S. J. Bennison et al., CeramicTransactions, 7 (1990) 13-49). To minimize the formation of spinel, thesolubility limit of magnesia in alumina preferably should not beexceeded.

The solubility limit of magnesia in alumina increases with increasingsintering temperature (see, R. L. Coble et al., J. Am. Ceram. Soc., 5[1] (1968) 1-6), increasing impurity level and is also affected by thesintering atmosphere. A reducing sintering atmosphere increases thesolubility limit and an oxidizing atmosphere, required to enhanceconductivity in accordance with this invention, decreases the solubilitylimit. When the solubility limit of magnesia is exceeded, a spinel phaseforms during sintering. In the process of the invention herein, it isnot essential to be below the solubility limit of MgO in Al₂ O₃.Electrical conductivity in the sintered body can be obtained at Mgadditions above the solubility limit. For the present invention,magnesia additions above the solubility limit can be tolerated withoutdegrading the electrical conductivity (preferably they do not exceedthree times the solubility limit). Magnesia contents at levels above thesolubility limit will not appreciably increase electrical conductivityor adversely affect electrical stability (i.e., the ceramic will exhibitessentially no resistivity drift at d.c. voltages of about 10 kV). Tooptimize conductivity, however, it is desirable not to exceed thesolubility limit since Mg addition above the solubility limit does notappreciably contribute to, and may degrade, electrical conduction. Notethat in the following discussion all magnesium cation (Mg²⁺) andchromium cation (Cr³⁺) additions are given as atomic mole fractions,i.e., mole of cation additive per mole of aluminum (Al³⁺) cation.

Normally, the sintering temperature will range from about 1550° C. toabout 1950° C. Generally in this range, the solubility limit of magnesiain alumina will vary from approximately 60 ppm to 3,300 ppm Mg²⁺ /Al³⁺as approximated by Coble et al., supra, using the following equation forsintering in a vacuum: 1nX=[8.1-(30,706/T)], where X is the atomic molefraction of Mg²⁺ /Al³⁺ and T is absolute temperature. The sinteringtemperature required to create fully dense bodies is preferably aboveabout 1550° C. and about 160 ppm Mg²⁺.

In the process of fabricating functional articles for the invention, useof high purity starting alumina and clean processing are important toavoid the formation of silicate-based grain boundary films. The startingalumina powders are often a source of impurities. The higher the purityof the starting alumina, the less impurities present and therefore thehigher the electrical conductivity. Generally the starting aluminashould be at least 99,900% pure and this purity level must be maintainedduring processing. Sumitomo AKP-30, Sumitomo Chemical America, New York,N.Y., (99.995% pure) which has a 0.5 mm crystallite size and 50% of theagglomerates below 1 mm, was found to be a suitable starting alumina.The powder is derived from an alkoxide route and although the metal ionimpurities are on the order of 10 ppm, the powder contains carbonresidue of the order of 100 ppm. Ceralox HP Al₂ O₃ was also found to bea suitable starting alumina.

Magnesia additions are necessary for creating electrical conduction.Magnesia suppresses the formation of liquid grain boundary phases duringsintering (see, Bennison et al., supra) which can degrade electricalproperties in polycrystals. Polycrystalline and single crystal (ruby)chromium/alumina samples without magnesia additions are electricallyinsulating. Also, the compositions must be fired in an oxidizingatmosphere to create electrical conductivity. Samples fired in inert orreducing atmospheres will be electrically insulating.

Additionally, in the process for preparing the magnesia/chromia/aluminaprecursor powder for fabricating functional articles of the invention,all processing should be carried out in clean conditions to avoidcontamination of the precursor powder and green body with unwantedimpurities. Ideally, glassware, metalware, and milling in ceramic millsshould be avoided in the preparation to minimize metal and silicaimpurities. Ball milling in ceramic mills should be avoided since it canintroduce unwanted silica impurities. The preparation process preferablyemploys Teflon or high density polyethylene (HDPE) containers, stirrers,and spatulas. If processing aids such as surfactants or binders areemployed, they should be of the clean-burning variety leavingessentially no residue on heat treatment. Surfactants and binders thatcontain metal ions should be avoided since these ions are adsorbedstrongly onto the starting powder and remain as unwanted impuritiesduring firing.

A suitable magnesia/chromia/alumina precursor powder for fabricatingfunctional articles can be prepared using a solution method where thechromium and magnesium cations are added as soluble metal salts to aliquid containing high purity alumina powder and other green processingaids. The cations can also be added as metal oxides into a liquidcontaining alumina powder. One preparation method can be accomplishedusing the following steps: (a) preparing a liquid containing a mixture(e.g., a solution and/or suspension) of the appropriate amount ofmagnesium and chromium cations by adding appropriate cation-containingprecursors to a solvent; (b) mixing with the liquid of step (a) a highpurity alumina powder (the alumina being preferably at least 99.900%pure, and more preferably at least 99.995% pure) to obtain awell-dispersed slurry; (c) evaporating the liquid from the slurry ofstep (b) to form a dried powder containing the magnesium and chromiumcations, alumina, and any other processing aids; (d) preparing a greenpowder compact from the powder of step (c); and (e) heating the greenpowder compact of step (d) in an oxidizing atmosphere to densify.

In preparing the liquid compositions of steps (a) and (b), any solventin which the magnesium and/or chromium precursor cation salts willdissolve or disperse can be used, such as methanol, ethanol, or water.The solvent should be high purity to minimize contamination. Themagnesium and chromium precursor salts can be those of nitrates,acetates, stearates, and chlorides or any other form that will decomposeupon sintering; or they can be oxides which effectively disperse uponsintering. The liquid can be evaporated by heating to form a driedpowder cake or by spray drying.

In fabricating functional articles, the magnesia/chromia/aluminaprecursor powder can be compacted to form a green body (compact) by anycompacting process yielding green densities of a least about 40%,preferably on the order of 55% or greater, of theoretical. During greenprocessing, clean burning binders should be employed in order tominimize sources of contamination. Appropriate compacting methodsinclude slip casting, tape casting, double-ended die pressing, isostaticpressing and injection molding. The method will vary depending on thefabricated article. For a forming method such as slip casting, castingcan be done directly from the precursor solution since the solublecations are strongly adsorbed onto the alumina surface and will not belost with the solvent into the casting mold. Slip casting can also beaccomplished by redispersing dried precursor powder in an appropriateliquid.

When sintering the green compact, it is preferable to surround thecompact with powder of the same composition and purity. This preventscontamination from the furnace and minimizes the loss of magnesia andchromia both which vaporize during heating. In order to maintain highelectrical conductivity, the sintering atmosphere must be oxidizing. LowO₂ partial pressure (e.g., reducing or inert atmospheres) results inpoor electrical conductivity. Preferably, the temperature is ramped at100° C./hr to 1000° C. and held for two hours. This effectively calcinesthe compact, decomposing the starting magnesium and chromium salts anddistributing the magnesium and chromium ions to the alumina, andremoving carbon impurities though pyrolysis and oxidation. The heattreatment is continued by ramping at 500° C./hr to the sinteringtemperature, for example 1650° C., with a one hour soak. The furnace isthen cooled to ambient. The resulting sintered article of the abovepreparation method has high density (approximately 99%), ispolycrystalline with a grain size of about 5 to 10 mm, and iselectrically conductive at room temperature.

Articles of this invention are useful as structural ceramic materials invarious applications. They exhibit superior mechanical properties suchas high strength, toughness and wear, and are especially useful inapplications requiring mechanical integrity. They have the addedadvantage of being about five times lighter than steel per unit volume.Oxygen-sintered articles of this invention further are particularlyuseful in electronic applications requiring selective electricalconductivity at or near room temperature, such as electrostatic chargingapplications. This invention provides a d.c. stable electronic conductor(i.e., an electronic conductor which has essentially no resistivitydrift). For example, an embodiment of this invention is useful for adischarge electrode in a process to produce flash-spun fibrous non-wovenstructures as described in U.S. Pat. No. 3,227,784 and U.S. Pat. No.3,387,326, which are hereby incorporated by reference herein.

U.S. Pat. No. 3,387,326 discloses an apparatus for spreading and thencharging a fibrous web in a corona charging field, and subsequentlydepositing the web uniformly in overlapping layers on a moving surfaceto form a nonwoven sheet. The corona charging field is generated by andconsists of an active charge generating electrode (e.g., an ion gun) anda passive charge collecting electrode (e.g., a target plate). The web isprepared by flash extrusion of a solution of crystallizable polymer. Inthis "flash extrusion" process, the strand is formed by extruding ahomogeneous solution of a fiber-forming polymer dissolved in a liquidwhich is a solvent for the polymer above its normal boiling point. Thesolution, at a temperature above the normal boiling point of the solventis extruded into a medium of lower temperature and substantially lowerpressure. The liquid within the extrudate then vaporizes and cools theextrudate, causing solidification of the polymer. The resulting fibrousweb may be spread out by causing it to impinge on a baffle or curvedsurface as it exits from the extrusion orifice. It may then be depositedupon a moving belt in overlapping, multidirectional layers to form anonwoven fibrous sheet.

A corona charging device is located along the path of advance justdownstream from the mechanical spreading device. It serves to place acharge on the fibers in the web and to thereby sustain them in aseparated or open condition. The charging device includes an electrode,such as an ion gun, and a second electrode, a target discharge plate.The target plate is a flat plate construction assembled in the equipmentwith the flat surface facing the ion gun and the path of advance. Thetrailing edge of the target plate terminates in a slightly rounded edge.

In optimizing a process to produce nonwovens, various modifications tothe target discharge plate can be contemplated (see, e.g., U.S. Pat. No.3,587,739, incorporated by reference herein, which discloses coveringthe target plate with an elastomer in a certain resistance range; andU.S. Pat. No. 3,860,369, hereby incorporated by reference herein, whichdiscloses incorporating the target plate and ion gun into an aerodynamicshield formed by two plates).

Target plates comprising the ceramic composition of this invention areuseful in such processes to produce fibrous webs because the ceramicexhibits desirable conductivity and mechanical properties. Accordingly,target plates of ceramics which are stable electronic conductors areprovided herein.

The use of a ceramic material having suitable resistivity for thepassive charge collecting electrode in accordance with this inventionimproves the operability, stability and the longevity of the chargingplate. A typical plate thickness is about 3/8 inch (0.95 cm).Preferably, the ceramic used for such plates has a volume resistivity offrom about 1×10⁹ to about 5×10⁹ ohm-cm measured at the operatingtemperature. For example, at 15 kV the ceramic used for a 0.95 cm thickplate suitably has a volume resistivity of about 1.6×10⁹ ohm-cm. Forthinner plates (e.g., down to about 0.18 cm) somewhat higher volumeresistivity (e.g., 5×10⁹ to 1×10¹⁰ ohm-cm) are generally more desirable.Thicker plates would use lower resistivities. It is also desirable thatthe ceramic have a Vickers hardness of at least about 5 GPa (preferablyat least about 14 GPa) and a modulus of rupture (i.e., breakingstrength) of at least about 400 MPa. Conductive ceramic compositions ofoxides of Mg⁺², Cr⁺³ and Al⁺³ of this invention are considered suitablefor this application.

The present invention can be further understood by reference to thefollowing non-limiting examples.

EXAMPLES

The following examples show that electrical conductivity occurred by agrain boundary conductivity mechanism, and was electronic in nature; andthat conductivity was affected by both Mg²⁺ and Cr³⁺ cation additionsand by silica impurities. All temperatures are expressed in degreesCelsius and all cation additions and impurities are given as atomic molefractions (i.e., mole of cation additive per mole of aluminum (Al³⁺)cation).

EXAMPLE 1

A precursor powder with cation additions of 500 ppm Mg²⁺ and 5000 ppmCr³⁺ was prepared in the following manner:

A. 126 mls of laboratory grade methanol were measured and placed in ahigh-density polyethylene beaker having a heated jacket.

B. 0.7451 g of 99.999% Cr₂ O₃ (Johnson Matthey Electronics, Orchard Rd,Royston, England), were weighed and added to the methanol. Thedispersion was treated to break-up agglomerates with an ultrasonic probeset at 40 W/cm² for 1 minute.

C. 0.3219 g of Mg(NO₃)₂.6H₂ O were added to the chromia/methanol mixtureand stirred until dissolved into solution.

D. 100 grams of Sumitomo AKP-30 alumina powder (Sumitomo ChemicalAmerica, New York, N.Y.) was weighed and added to themagnesia/chromia/methanol solution.

E. The dispersion was again treated to break-up agglomerates with anultrasonic probe for 1 minute.

F. The mixture was heated to 40° C. and stirred with a HDPE stirreruntil the slurry thickened through evaporation of the solvent.

G. The stirrer was removed and the remaining solvent evaporated off toyield the precursor power.

H. When dry, the precursor powder was crushed to break-up softagglomerates and sieved through a 325 mesh screen.

I. The precursor powder was placed in a double ended die and formed into1.25 inch (3.18 cm) diameter pellets using 3500 psi (24.1 MPa) ofuniaxial pressure.

J. The resultant pellets were placed in a bed of unpressed precursorpowder and placed in a covered high purity alumina crucible. Theassembly was then placed into an electric furnace and sintered in air to1650° C. as follows; 2° C./min. up to 1000° C. and held at 1000° C. for2 hours; 8° C./min. to 1650° C. and held at 1650° C. for 1 hour; thencooled to ambient. The resultant sintered samples were approximately 1.0inch (2.54 cm) in diameter and approximately 0.125 (0.32 cm) inchesthick, dense and flat with no cracks, ruby in color, and were slightlytranslucent.

Electrical measurements were made on 1.0 inch (2.54 cm) diametersintered sample disks by a two-probe d.c. method (Examples 1-16). Twoprobe d.c. measurements were used to calculate volume resistivities andthe ac impedance measurements were used to characterize electricalconductivity mechanisms. The d.c. measurement data are expressed asvolume (bulk) resistivity (Ωcm).

For the two-probe d.c. technique, sample disks were not electroded sincenonohmic contact behavior is not a concern when measuring resistivitiesof the magnitude of these compositions. A constant voltage was impressedacross the samples using 0.25 inch (0.64 cm) diameter copper contactsplaced on the top and bottom surfaces of the sample disks. The resultantcurrent through the sample was then measured. Voltage and currentmeasurements were made using a Hitotronics H800-PL unit. Voltage wasvaried from 1 kV to 10 kV to characterize voltage effects. Measurementswere made in a constant temperature and humidity chamber at roomtemperature and 60° C., and 50% humidity.

EXAMPLES 2-16 Volume Resistivity Measurements

Examples 2 through 16 were prepared essentially as described in Example1, however the amounts of the components were varied. The actual amountsused and the resistivity measurement (d.c. method) for each sample areshown in Table I. Additionally, Examples 6 through 16 were tested forelectrical stability (resistivity drift) for a 24 hour time period.

FIG. 1 plots results from Examples 1-5 and shows the effect of chromiumcation (Cr³⁺) additions on volume resistivity at constant voltage (10kV), temperature (60° C.), and magnesium cation (Mg²⁺) content (500ppm). This figure shows that resistivity decreased from insulating toless than 10⁸ Ωcm as the amount of chromium cations were increased. Thedata was fitted to the solid line using the method of least squares. Thefollowing empirical equation was generated from the force fit data at500 ppm Mg²⁺ : ρ=a X^(m), where ρ is the volume resistivity, X is themole fraction of Cr³⁺ cations, a is a constant which equals 1.82×10¹⁰,and m is an exponent which equals -3.86. Substituting a and m into thisequation gives ρ=1.82×10¹⁰ X⁻³.86. This equation can be used todetermine the effect of chromium cation additions on resistivity and isuseful for functional design.

FIG. 2 depicts Examples 2-10 and shows the effect of chromium cationadditions on volume resistivity at constant voltage (10 kV), temperature(60° C.), and magnesium cation content (700 ppm). This figure shows thatresistivity decreases as the amount of chromium cations were increased.Comparing FIGS. 1 and 2, it is evident that samples with 500 ppmmagnesium cation were more resistive than similar samples with 700 ppmmagnesium cations. (FIG. 4 shows the effect of magnesium cationadditions on resistivity.) The following empirical relation wasgenerated from the fitted data at 700 ppm Mg²⁺ : ρ=1.29×10¹⁰ X⁻⁴.05.

FIG. 3 depicts Examples 2-5 and shows the effect of voltage on volumeresistivity for various chromium cation additions at constanttemperature (60° C.), and magnesium cation content (500 ppm). Thisfigure shows that resistivity increased as the d.c. voltage wasdecreased. The figure also shows that resistivity decreased as theamount of chromium cations were increased, as discussed above.

FIG. 4 depicts Examples 11-16 and shows the effect of magnesium cationadditions on volume resistivity at constant voltage (10 kV), temperature(60° C.), and chromium cation content (20000 ppm). This figure showsthat resistivity decreased from insulating to less than 10⁹ as theamount of magnesium cations were increased. The following empiricalrelation was generated from the fitted data: ρ=5.61×10⁹ X⁻⁰.98. Notethat at magnesium additions greater than about 700 ppm there was nofurther decrease in resistivity. The solubility limit of magnesia inalumina is approximately 500 ppm. Magnesia addition above the solubilitylimit does not appreciably contribute to conductivity, and may degradeconductivity.

Electrical stability testing at high d.c. voltage (10 kV) for Examples6-10 are shown in FIG. 5 which plots resistivity versus time for variouschromium cation additions at constant temperature (60° C.) and constantmagnesium cation content (700 ppm). This figure shows that resistivitydid not drift or increase towards voltage breakdown with time. Thesesamples were electrically stable at 10 kV under d.c. conditions for 24hours, which was the test duration. FIG. 6 shows a similar graph forvarious magnesium cation additions at constant temperature (60° C.) andconstant chromium cation content (20000 ppm), which are Examples 12-16.This figure shows that samples with low magnesium cation additions (lessthan about 100 ppm) were not as electrically stable with time(resistivity increased with time) as the samples with higher (greaterthan about 100 ppm) magnesium cation additions. Electrical instabilityis shown as an increase in resistivity with time which eventually leadsto voltage breakdown. Electrical stability is especially important inhigh voltage d.c. applications and is generally not obtainable inelectronic ceramics to the degree observed in the subject materials.

EXAMPLE 17 Target Discharge Plate

A ceramic was prepared substantially similar to the manner of Example 1,except the ceramic was formed into a functional electrostatic dischargeplate. The precursor powder was placed in a mold and uniaxially drypressed at 3500 psi to form an unfired circular plate which was 0.5inches thick and 12.0 inches in diameter. This plate was then sinteredin air at 1650° C. The resulting plate had an outer diameter dimensionof 9.84 inches due to shrinkage of approximately 18%.

EXAMPLE 18 Fiber Charging

A target discharge plate prepared substantially in accordance with theprocedure of Example 17 was machined, forming an annulus having a 4.1inch (10.4 cm) inside diameter and a 9.0 inch (22.9 cm) outside diameterto conform to the desired dimensions of a target plate for flashspinning. The target plate surface was polished to about a 9-microinchsurface roughness with a resulting thickness of 0.375 inches (0.95 cm).A 45 degree chamfer, about 1/16 inch (0.16 cm) wide with rounded edges,was machined on the outside edge. A metallic paint on the back of theplate was used for attachment to ground. The volume resistivity of thisfinished target plate was about 5×10⁹ ohm-cm. The Vickers hardness ofthe discharge plate ceramic is estimated as above 14 GPa.

The ceramic target plate was compared to a metal target plate in atwo-position test spin where a 20 weight percent solution of highdensity polyethylene in hydrocarbon solvent was flash spun at about 55pounds per hour (25 Kg/h) substantially in accordance with the generalprocedures outlined in U.S. Pat. No. 3,227,784, U.S. Pat. No. 3,387,326,U.S. Pat. No. 3,578,739 and U.S. Pat. No. 3,860,369. The flash-spun webswere (1) passed through the charging zones formed by an ion gun andceramic plate in one position and an ion gun and metal plate in a secondposition, (2) then passed through identical diffusers, and (3) laid andpinned to a belt moving at 50 yards a minute (45.7 m/min.). Typically,during the spin, the ion gun potential and current were maintained atabout 10 kV (direct current) and about 350 microamps. The web currentwas about 40 microamps on the position with the ceramic plate and about45 microamps on the position with the metal plate, giving chargingefficiencies of about 11 and 13 percent, respectively, with a resultingweb charge of about 6 microcoulombs/gm in both cases. Both webs werejudged to pin both equivalently and satisfactorily to the belt. However,the ceramic target plate resistance, at about 5×10⁹ ohm-cm, was judgedto be somewhat higher than optimum as there was some leakage currentaround the outer surface to ground. Discharge plates of somewhat lowerresistance can readily be prepared in accordance with this invention.

EXAMPLE 19 AC Impedance Measurements--Electrical Conductivity Mechanisms

Five samples were selected for a.c. impedance characterization to showthe effects of magnesium and chromium cation additions and silicaimpurities on electrical properties. These examples show that electricalconductivity occurred by a grain boundary conductivity mechanism and waselectronic in nature, and that conductivity was affected by both Mg⁺²and Cr⁺³ cation additions and by silica impurities.

Quantitative elemental analyses as determined by SSMS (spark source massspectrometry, Northern Analytical Laboratories, Merrimack, N.H.) for thefive samples are listed in Table III. The samples included Example 4(prepared as above) and four Comparative Examples (A, B, C, and D). Thefive samples, characterized according to their major elementalcomponents were as follows: Example 4--30,000 ppm Cr³⁺ /500 ppm Mg²⁺/alumina with no added silica impurities; Comparative Example A--30,000ppm Cr³⁺ /500 ppm Mg²⁺ /alumina with 1000 ppm silica; ComparativeExample B--20,000 ppm Cr³⁺ /500 ppm Mg²⁺ /alumina with 1000 ppm silica;Comparative Example C--500 ppm Mg²⁺ /alumina with no chromium or addedsilica; Comparative Example D--single crystal ruby (500 ppm Cr³⁺).

Examples 4 and A-D were prepared by the solution method, as described inExample 1, to minimize impurities. In addition to the solutionpreparation, Examples A and B were ball milled in alumina mills foreight hours. Ball milling effectively introduced silica and otherimpurities, as indicated by the elemental analysis (Table III), andshould be avoided. Silica impurities were expected to degrade electricalconductivity. Example D, single crystal ruby, was electronic grade(Union Carbide, Crystal Products Division, Washougol, Wash.) withminimal impurities.

AC impedance measurements were used to characterize electricalconductivity mechanisms. In the complex impedance approach, the overallelectrical response of a material can be modeled in terms of equivalencecircuits that distinguish individual processes contributing toconductivity within the material. Each conductivity component isassigned a parallel resistor-capacitor network that are connected inseries as shown in FIG. 7, a complex impedance plot for Example 4. Ifthe capacitance of each network is sufficiently different, a series ofcontacting semicircles can be obtained in a complex impedance plotallowing individual conductivity processes to be distinguished.Impedance plots are useful for characterizing bulk, grain boundary, andbulk/electrode interface responses. It is also effective forcharacterizing any changes in conductivity processes that can occur fromvariations in processing.

For the a.c. impedance measurements, sample disks were electroded withevaporated gold metal on the top and bottom surface's to ensure ohmiccontacts. Two terminal measurements of the capacitance and loss tangentwere made in the frequency range from 13 Hz to 100 Hz using a GenradGR1689 bridge. Measurements at room temperature were obtained using aHewlett-Packard 16043E test fixture. Measurements taken at hightemperature were done in a quartz tube furnace using a test fixture madeof high-purity alumina and sapphire. Prior to all measurements, shortand open calibrations were done for the instruments and the holders ateach measurement temperature and compensations were included in theresultant calculations. AC impedance measurement data are expressed asbulk conductivity ((Ωcm)⁻¹).

FIG. 7 shows a complex impedance plot at 250° C. for Example 4, aluminawith 30,000 ppm Cr³⁺ and 500 ppm Mg²⁺, with no silica. The figure showstwo well formed connecting semicircles corresponding to components ofhigh and low frequency conduction. Both semicircles correspond toregions of grain boundary electrical conductivity. Bulk conductivity inalumina based materials is highly unlikely at these temperatures.

FIG. 8 shows an arrhenius plot in the temperature range of ambient to600° C. for the low and high frequency components of conductivity forExample 4. The log σT--1/T plots are linear. Using a method of leastsquares fit and the Arrehnius relation, σT=σ_(o) e.sup.(-E/KT), where σis conductivity, σ_(o) is the pre-exponential, E is the activationenergy, K is the Boltzman constant, and T is absolute temperature,activation energies of 0.46 eV (44.4 kJ/mol) and 0.60 eV (57.9 kJ/mol)were calculated for the respective low frequency (Q₁) and high frequency(Q₂) conductivity components. Activation energies in this range andtemperature region are consistent with electronic conductivity. Ionic orfast ion conductivities are highly unlikely in alumina-based materialsat these temperatures.

Similar complex impedance plots and activation energies can be generatedfor the Examples A-D listed in Table II. These results are summarized inTable II which shows electrical resistivities measured at 25° C., 125°C. and 350° C. and the respective low and high frequency activationenergies. Example 4 without silica impurities was more conductive andhad lower activation energies than Examples A and B with silicaimpurities. Clearly silica degrades electrical conductivity. Example C(no chromium) was substantially more insulating with a higher activationenergy than samples containing both chromium and magnesium. Example D,single crystal ruby (no magnesium), was insulating. Clearly chromium andmagnesium are required to create electrical conductivity. Grain boundaryconductivity is further reinforced by the observation that the singlecrystal ruby was insulating suggesting that the lack of grain boundariesin the single crystal inhibits conductivity. Also note that in Example B(20000 ppm Cr³⁺ /500 ppm Mg²⁺ /alumina with silica) the high frequencycomponent to conductivity was absent and thus only the low frequencyactivation energy was calculated. The absence of a high frequencyconductivity component, as well as higher low frequency activationenergies and higher resistivities, suggests that silica impurities wereinhibiting conductivity by deleteriously altering conductive pathwaysalong grain boundaries. Example C with no chromium also lacked a highfrequency component which suggests a similar degradation in conductivityfrom lack of chromium additions.

In summary, the complex impedance analysis suggests that chromium andmagnesium additions in alumina in optimized amounts enhanced electricalconductivity by creating effective pathways for conduction along grainboundaries. Silica, and other impurities, inhibited conductivity bydegrading these conductive pathways.

                                      TABLE I                                     __________________________________________________________________________    Volume Resistivity, ρ                                                     (60° C., 10 kV, 60 min.)                                                  MeOH                                                                              Al.sub.2 O.sub.3                                                                  Cr.sub.2 O.sub.3                                                                  MgNO.sub.3                                                                         Cr  Mg r    r     SEE                                     EX (g) (g) (g) (g)  (ppm)                                                                             (ppm)                                                                            DRIFT*                                                                             (ohm-cm)                                                                            FIGS.:                                  __________________________________________________________________________    1  159.54                                                                            100.0                                                                             0.7451                                                                            0.3219                                                                             5000                                                                              500                                                                              --   3.5 × 10.sup.11                                                               1                                       2  159.54                                                                            100.0                                                                             1.4902                                                                            0.3219                                                                             10000                                                                             500                                                                              --   9.7 × 10.sup.10                                                               1,3                                     3  159.54                                                                            100.0                                                                             2.9804                                                                            0.3219                                                                             20000                                                                             500                                                                              --   1.6 × 10.sup.9                                                                1,3                                     4  159.54                                                                            100.0                                                                             4.4706                                                                            0.3219                                                                             30000                                                                             500                                                                              --   4.1 × 10.sup.8                                                                1,3,7,8                                 5  159.54                                                                            100.0                                                                             5.9608                                                                            0.3219                                                                             40000                                                                             500                                                                              --   6.2 × 10.sup.7                                                                1,3                                     6  159.54                                                                            100.0                                                                             0.7451                                                                            0.4506                                                                             5000                                                                              700                                                                              None 2.5 × 10.sup.11                                                               2,5                                     7  159.54                                                                            100.0                                                                             1.4902                                                                            0.4506                                                                             10000                                                                             700                                                                              None 1.1 × 10.sup.10                                                               2,5                                     8  159.54                                                                            100.0                                                                             2.9804                                                                            0.4506                                                                             20000                                                                             700                                                                              None 6.5 × 10.sup.8                                                                2,5                                     9  159.54                                                                            100.0                                                                             4.4706                                                                            0.4506                                                                             30000                                                                             700                                                                              None 1.7 × 10.sup.8                                                                2,5                                     10 159.54                                                                            100.0                                                                             5.9608                                                                            0.4506                                                                             40000                                                                             700                                                                              None 5 × 10.sup.8                                                                  2,5                                     11 159.54                                                                            100.0                                                                             2.9804                                                                            0.0322                                                                             20000                                                                             50 Yes  3. × 10.sup.10                                                                4                                       12 159.54                                                                            100.0                                                                             2.9804                                                                            0.0644                                                                             20000                                                                             100                                                                              Yes  4.2 × 10.sup.9                                                                4,6                                     13 159.54                                                                            100.0                                                                             2.9804                                                                            0.1287                                                                             20000                                                                             200                                                                              Yes  1.8 × 10.sup.9                                                                4,6                                     14 159.54                                                                            100.0                                                                             2.9804                                                                            0.3219                                                                             20000                                                                             500                                                                              None 1.6 × 10.sup.9                                                                4,6                                     15 159.54                                                                            100.0                                                                             2.9804                                                                            0.4506                                                                             20000                                                                             700                                                                              None 6.5 × 10.sup.8                                                                4,6                                     16 159.54                                                                            100.0                                                                             2.9804                                                                            0.6437                                                                             20000                                                                             1000                                                                             None 8.1 × 10.sup.8                                                                4,6                                     __________________________________________________________________________     *24 hour test                                                            

                                      TABLE II                                    __________________________________________________________________________    Summary of a.c. impedance measurements                                        EX Composition (ppm)                                                                          25° C. (Ωcm)                                                            125° C. (Ωcm)                                                            350° C. (Ωcm)                                                            Q.sub.1 (eV)                                                                       Q.sub.2 (eV)                      __________________________________________________________________________    4  30000Cr/500Mg/Al.sub.2 O.sub.3                                                             1.5 × 10.sup.9                                                                 7.7 × 10.sup.7                                                                  1.1 × 10.sup.6                                                                  0.46 0.60                                 no added Si                                                                A  30000Cr/500Mg/Al.sub.2 O.sub.3                                                             >10.sup.12                                                                           1.1 × 10.sup.11                                                                 9.1 × 10.sup.7                                                                  0.80 0.86                                 1000Si                                                                     B  20000Cr/500Mg/Al.sub.2 O.sub.3                                                             >10.sup.12                                                                           1.4 × 10.sup.11                                                                 3.7 × 10.sup.8                                                                  0.87 --                                   1000Si                                                                     C  500Mg/Al.sub.2 O.sub.3 no added Si                                                         >10.sup.12                                                                           >10.sup.12                                                                            7.1 × 10.sup.9                                                                  1.40 --                                D  Ruby Single Crystal                                                                        >10.sup.12                                                                           >10.sup.12                                                                            >10.sup.12                                                                            --   --                                __________________________________________________________________________

                  TABLE III                                                       ______________________________________                                        Quantitative elemental Analysis as determined by SSMS                         4            A       B         C     D                                        ______________________________________                                        Li     0.25      5.0     5.0     0.25  0.1                                    Na     25.0      500.0   300.0   5.00  1.5                                    Mg     500.0     500.0   500.0   500.00                                                                              2.5                                    Cr     Major     Major   Major   0.4   225.0                                  Si     10.0      1000.0  1000.0  7.50  7.5                                    K      1.0       75.0    75.0    1.00  0.5                                    Ca     2.5       75.0    50.0    2.00  1.0                                    Ti     0.05      15.0    5.0     0.05  0.05                                   Fe     2.5       15.0    10.0    1.50  0.5                                    ______________________________________                                    

What is claimed is:
 1. An apparatus for electrostatically charging acontinuous fibrous material being forwarded in a path from a source to acollecting means including a charged ion gun and an opposed groundedtarget electrode positioned on opposite sides of said path between saidsource and said collecting means, wherein:said target electrode has aceramic surface facing said ion gun, said ceramic surface being formedof a stable electronic conductor comprising a polycrystalline, solidsolution alumina-based ceramic composition having a volume resistivityof from about 1×10⁷ to 5×10¹⁰ ohm-cm measured at the operatingtemperature and a Vickers hardness of at least about 5 GPa.
 2. Theapparatus of claim 1 wherein the ceramic composition has a Vickershardness of at least about 14 GPa.
 3. The apparatus of claim 1 whereinthe ceramic composition consists essentially of oxides of Mg²⁺, Cr³⁺ andAl³⁺, wherein the atomic ratio of Cr³⁺ to Al³⁺ is within the range offrom about 1000 to about 100,000 ppm, and the atomic ratio of Mg²⁺ toAl³⁺ is within the range of from about 25 ppm to 10,000 ppm.